Displays such as liquid crystal displays (LCDs) and organic LED displays (OLEDs) occupy a significant part of the surface of many electronic devices such as mobile telephones, televisions and monitors. Photovoltaic power generation devices also are used in some of these devices and must occupy large areas to generate significant amounts of power.
Liquid Crystal Displays
FIGS. 1a and 1b show schematic diagrams of typical active-matrix liquid crystal displays. FIG. 1a is a colour transmissive display, and FIG. 1b is a monochrome reflective display.
In each case, the display is constructed on two substrates 1 and 2. These are typically glass but may also be polymer. The liquid crystal layer 10 is a nematic liquid crystal in the case of the transmissive display, and a polymer/liquid crystal composite in the case of the reflective display. In both cases, a common electrode 5 on the upper substrate, made of a transparent conductor such as indium tin oxide (ITO), and a grid of pixel electrodes 9 on the lower substrate are used to apply voltage to the liquid crystal layer. Thin-film transistors (TFTs) 7 are used to control the pixel electrodes, and a so-called ‘via’ connection 8 is made through a passivation and planarisation layer 13 to the pixel electrodes 9.
In the transmissive display (a), crossed external polarisers 3 are placed on the outer surfaces of the substrates. The pixel electrodes 9 are also transparent. Applying voltage to the liquid crystal layer changes the orientation of the liquid crystal molecules, and this affects the polarisation of light passing through the display from the backlight 11. In states where the polarisation is unchanged, the pixel appears black, and in states where the polarisation is changed, light is transmitted and the pixel appears bright. The colour-filter substrate 2 has on its inner surface red, green, and blue colour filters 4. Alignment layers 6 (typically polyimide) control the alignment of the liquid crystal molecules. Examples also exist where the polarisers are oriented with their transmission directions parallel, so that when the polarisation is unchanged, the display appears white.
Reflective liquid crystal displays can be constructed using a wide variety of different methods. In the reflective display example shown in FIG. 1b, the pixel electrodes 9 are typically made of specularly reflective metal but may also be made of a light absorbing conductor, which for example could be made from a layer of a transparent conductor together with a black mask underneath the conductor. The liquid crystal layer 10 consists of a polymer network and regions of liquid crystal. With voltage applied, the molecules in the liquid crystal orient so that there is little refractive index contrast between the two types of domain, and the liquid crystal layer is transparent. With specularly reflective electrodes used as pixel electrodes, the appearance of the pixel is similar to that of a mirror, and it appears dark unless a light-source happens to be in the correct position for its light to reflect specularly into the viewer's eyes. In the case of the pixel electrodes made of light absorbing conductor, the appearance of the pixel is dark. With no voltage applied, the liquid crystal domains orient randomly, and the resulting index contrast causes scattering of light. The pixel appears bright.
This type of display is known as a polymer network liquid crystal display (PN-LCD). Details of the technology are given by H. Hasebe et al in the Journal of photopolymer science and technology, volume 10(1) pp 25-30 (1997) and by T. Fujisawa et al, in the same journal, volume 11(2), pp 199-204 (1998). A display of this type with reflective pixel electrodes is described, for example, by Y. Asaoka et al in SID Symposium Proceedings 2009 paper 29.1. The paper by Asaoka et al also describes how memory elements may be incorporated into each pixel, in order to reduce the power required to sustain an image on the display.
A PN-LCD display with a reflecting layer behind the liquid crystal cell appears brighter than one with a dark layer behind, because light which is scattered to the reflector can be reflected back to the scattering layer and either transmitted or scattered again to reach the viewer's eyes. This brightness improvement is important because to achieve a driving voltage low enough for operation with an active matrix (TFT) driving circuit, the PN-LCD layer must be made with a small thickness (of order 5 μm). This small thickness leads to relatively weak scattering, and little back-scattering. In this case, most of the scattered light strikes the reflecting layer, and so the reflecting layer is necessary for high brightness. However the reflecting layer has the disadvantage of increasing the brightness of the dark state, and therefore reducing the contrast.
In the PN-LCD displays mentioned above, the PN-LCD layer scatters light when no voltage is applied, and is transparent when voltage is applied. It is also possible to make a PN-LCD display where the zero-voltage state is transparent, and applying a voltage causes scattering. Examples are U.S. Pat. No. 7,102,706 (Kim; Boe Hydis), U.S. Pat. No. 5,188,760 (Hikmet; Philips), and the article by Sonehara et al in SID Symposium Proceedings 1997 pp 1023-6.
For simplicity, there are a number of components omitted from these diagrams, including insulation layers, other features used to align the liquid crystal, addressing circuits and black masks. Also the details of construction vary from one type of display to another.
Other Scattering-Mode Reflective Displays
Other methods exist for making a display pixel switch between scattering and transparent states. Polymer-dispersed liquid crystals (PDLCs) are well known (see, for example, L. Bouteiller and P. Le Barry, ‘Polymer-dispersed liquid crystals: preparation, operation and application’, Liquid crystals vol 21, pp 157-174 (1996)). In these devices, droplets of liquid crystal are dispersed in a polymer matrix. However these devices normally operate at voltages too high to be compatible with active-matrix (TFT) driving circuits.
In addition to the polymer-network liquid crystal displays described above, there are some other methods of making a liquid crystal scattering display with low driving voltage. An example is given in U.S. Pat. No. 5,539,556 (Demus; Chisso, Inc.), where droplets of liquid crystal are suspended in a fluid which does not mix with the liquid crystal.
There are also ways of making scattering-mode reflective displays which do not use liquid crystals at all, for example, electro-wetting and in-plane electrophoretic displays. In the most common electro-wetting geometry, the device consists of a mixture of two immiscible fluids, one of which is black, and the other of which is transparent (see, for example, R. A. Hayes & B. J. Feenstra, “Video-speed electronic paper based on electrowetting”, Nature, Vol. 425, pp 383-385 (2003)), Situated behind the fluids is a scattering white reflector. Since it is possible to control the position and area occupied by the black fluid, it is therefore possible to either cover with black, or reveal the white scatterer to the viewer within any one particular pixel, thereby creating what effectively amounts to a scattering-mode display, although none of the fluids changes between scattering and transparent (they are merely moved about as required). A less conventional way to achieve the same effect would be to use electro-wetting fluids which were transparent and white, with a black background (rather than transparent and black, with a white background). Likewise, similar effects can be achieved using black or white electrophoretic particles suspended in a transparent fluid.
Photovoltaic Elements
One type of known photovoltaic element is a thin film solar cell. A review of thin-film solar cell technology can be found in the paper by K. L. Chopra, “Thin-Film Solar Cells: An Overview”, Prog. Photovolt., Res. Appl., 2004, vol 12, pp 69-92. Thin film solar cells comprise a plurality of thin layers (or films) of materials, typically 10 nm-10 μm in thickness, deposited in a stack on a supporting substrate 17 (FIG. 2a). The thin film stack typically comprises at least two conducting layers 16 and 14, and a light absorbing layer 15. A thin film solar cell, like other types of solar cells, exploits the properties of a light absorbing layer to convert light energy to electricity by the photovoltaic effect. Free charge carriers generated from the absorption of photons in the light absorbing layer are transported under the influence of an internal potential gradient to the conducting layers where they are collected and used to power an external circuit.
The construction of the solar cell is designed so as to accept and absorb as much incident light as possible for maximum efficiency. At least one of the conducting layers 16, known as the front contact, must allow the incident light 19 to pass to the light absorbing layer. For example, this is achieved if the conducting layer is patterned so that regions within the layer have no conducting material. The conducting material may be arranged in stripes which are known as finger contacts. Alternatively, the conducting layer may be transparent to light of the required wavelength range 16a. Examples of common transparent conducting layers are Indium Tin Oxide (ITO), Fluorine doped Tin Oxide (SnO2:F), or aluminium doped zinc oxide (ZnO:Al).
Optionally the front contact may comprise two or more layers. For example, finger or grid contacts 16b and a transparent conducting layer 16a can be combined 16 to achieve optimum light transmission and minimize sheet resistance and light absorption and reflectivity of the conducting layer. In another example additional layers may be added to minimize reflections or increase light trapping. The front contact may include further additional features to optimise its electrical or optical properties. For example, the interface between the conducting layer and the light absorbing layer may be roughened to prevent reflections and promote light trapping as described in the paper by Zeman et al., “Optical modeling of a-Si:H solar cells with rough interfaces: Effect of back contact and interface roughness”, J. Appl. Phys., vol 88 (11), 2000, pp 6436-6443.
The second conducting layer 14, known as the back contact, is typically reflective in order to increase the path length of the incident light in the light absorbing layer. Typical materials that are used for the reflective back contact are silver, chromium, or aluminium. The back contact may also comprise one or more additional features to scatter the reflected light and further increase the path length in the light absorbing layer. For example, the back contact 14 may combine a reflective metal 14a with an additional transparent layer 14b such as aluminium doped Zinc Oxide (ZnO:Al) designed to promote light scattering. One or more of the back contact layers may be subjected to processes, such as etching, designed to increase its roughness and further promote scattering.
Typically the light absorbing layer is an inorganic semiconductor such as silicon, although many other materials can be substituted, including organic semiconductors, dyes and electrolytes. Typically the light absorbing layer will include at least one semiconductor junction between sub-layers of different electrochemical potential in order to create an internal potential difference. A semiconductor junction is typically obtained by combining two materials of different composition. Variations in material composition can be achieved by differently doping the same semiconductor or by combining different materials. The light absorbing layer 15 in silicon thin film solar cells typically comprises thin highly p-doped 15c and n-doped 15a sub-layers either side of a thicker intrinsic (i-) silicon layer 15b to form a p-i-n junction.
Thin film solar cells are typically constructed with one of two types of configuration. A “substrate configuration” (FIG. 2a) is defined when the supporting substrate is adjacent to the back contact. In this configuration the substrate is not required to be transparent. Alternatively, a “superstrate configuration” (FIG. 2b) is defined when the supported substrate 18 is adjacent to the front contact 16c. In this case the substrate is required to be transparent to allow light to enter the solar cell.
Incident light is not absorbed or is weakly absorbed if the photon energy is smaller than the band gap of the light absorbing layer. On the other hand, if the photon energy is greater than the band gap, the excess energy will be lost in the form of heat. To increase the efficiency of the solar cell, additional light absorbing layers 20 composed of materials with different photoelectric properties may be stacked on top of each other (FIG. 3). As with the first light absorbing layer 15, each sub-cell in the stack also comprises n-doped 20a, intrinsic 20b and p-doped 20c sub-layers, but absorbs a different part of the solar spectrum; hence the overall absorption of the solar cell and therefore the efficiency is increased. This type of thin film solar cell with two or more light absorbing layers is known as a multijunction thin film solar cell. Examples of commonly combined materials for multijunction silicon thin film solar cells are amorphous silicon, microcrystalline silicon, silicon carbide, and silicon-germanium, although other materials and combinations are well known. A review of silicon-based thin-film technology can be found in the paper by A. V. Shah et al, Prog. Photovolt. Res. Appl., 2004, vol 12, pp 113-142.
Photovoltaic functions can also be achieved by thin films of polymers. A recent review summarises these devices. (‘Polymer based photovoltaics: novel concepts, materials and state-of-the art efficiencies’, J. M. Kroon et al, Proceedings of the 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona 2005). Polymer-based PV cells show much lower efficiency than inorganic PVs, but have the advantage of lower-cost production processes, including solution processing (for example, see Wang et al, Applied Physics Letters vol 95, 043505 (2009)).
In addition to silicon and polymers, thin film solar cells based on other materials exist. Well known examples are based on Copper Indium Gallium Selenide (CIGSe2), Copper Indium Gallium Sulfide (CIGS2), Cadmium Telluride (CdTe), Any of these or other thin film solar cell technologies may be preferred on the basis of their intended application/function, efficiency, cost, environmental impact, process compatibility, wavelength range of operation, or aesthetic (e.g. colour/appearance). For an overview of different thin film solar cell technologies see Thin Film Solar Cells: Fabrication, Characterization and Applications, Ed. J. Poortmans and V. Arkhipov, published by Wiley, 2006.
The construction of many thin film solar cells is such that additional layers are required between the incident light and the light absorbing layer to improve the overall performance of the cell. Such layers (sometimes known as window layers) do not contribute to the photocurrent. Furthermore, the bandgap of the window layers is chosen to be larger than that of the light absorbing layer in order to allow light to pass through. However, the window layer will absorb photons with energies greater than its band gap, and therefore prevent the photons from passing through to the light absorbing layer. For example, a CdS buffer layer is commonly used in the case of CIGS and CdTe thin film solar cells. CdS absorbs light of wavelengths shorter than approximately 500 nm. ZnO and ITO are commonly used as the transparent conducting layers but also typically absorb light of wavelengths less than approximately 350 nm. As a result of these factors thin film solar cells typically only make efficient use of a region of the solar spectrum between a lower limit set by the band gap of the light absorbing layer and an upper limit set by any additional windows layers in the cell. For a description of the light absorbing properties of several common thin film solar cells see Jenny Nelson, The Physics of Solar Cells, Imperial College Press, 2003, page 213-214. Figures showing the external quantum efficiency of some thin film solar cell technologies can be found in various publications, including: for amorphous silicon, Meier et al., “High-efficiency amorphous and micromorf silicon solar cells”, Proc WCPEC 3, 2003, S20-89-06; for CIGS, Kaigawa, “Improved performance of thin film solar cells based on Cu(In,Ga)S2”, Thin Solid Films, 2002, 415, p 266; and for CdTe, A. D. Compaan, MRS Symp. Proc., 2004, 808, A7.6.
During fabrication of large area thin film solar cells (typically greater than 1 cm2), a single solar cell may be subdivided into many smaller solar cells to form a solar module. The individual cells are typically approximately 1 cm wide, although their length may vary, and are connected in series to the adjacent cells forming a cascade of cells. The sub-division of large cells into smaller cells and their connection series reduces the current output produced by each cell thereby reducing Joule losses due to the sheet resistance of the conducting layers. Series connection also allows the total output voltage of the cells in the cascade to be increased to a more suitable level for the external circuit to which they are connected.
Series connections are typically achieved by “monolithic contacts” between adjacent cells. Monolithic contacts made by a sequence of three cutting steps, each of which cuts through different layers in the cell in order to connect the back contact of one cell to the front contact of the adjacent cell (FIG. 13). The first cut 42 is made after the back contact is deposited and separates the back contacts of neighbouring cells (FIGS. 11a & b). The second cut 43 is made after deposition of the light absorbing layer and is off-set from the first cut (FIGS. 11c & d). The second cut separates the light absorbing layer and opens a via so that the front and back contacts of adjacent cells can be electrically connected. The third cut 44 is made after deposition of the front contact layers and separates the front contact of neighbouring cells (FIGS. 11e & f). The cuts are commonly performed by laser scribing, although lithographic methods can also be employed. A detailed description of this laser scribing process can be found here: C. M. Dunsky, Industrial Laser Solutions, February 2008. (http://www.coherent.com/Downloads/80410-038e-ILS.pdf).
FIG. 14b shows a simple equivalent circuit diagram for one solar cell including the monolithic contact region. By common convention the solar cell is represented by a current source and diode in parallel. The resistors R1 to R5 (excluding R3) approximate the series resistances arising from the front and back contact layers. R3 is a shunt resistance which accounts for leakage between neighbouring back contacts through the n-type a-Si layer. FIG. 15 shows an exemplary circuit diagram of a module containing six solar cells monolithically connected in two parallel cascades of three cells each. A detailed description of the monolithic contacts can be found in the paper by Brecl and Topic, “Simulation of losses in thin-film silicon modules for different configurations and front contacts”, Prog. Photovolt: Res. Appl., 16, 2008, pp 479-488.
Monolithic contacts reduce the active area 41 of the cell since they produce a dead region 40 around the contact area which does not contribute to the photocurrent (FIG. 13). The cell width must be optimized to balance the loss of active area 40 arising from having more contacts with reduced Joule losses arising from the lower current passing through the cells. A detailed description of the losses and their relationship to the cell and interconnect dimensions is provided by Gupta et al., “Optimisation of a-Si solar cell current collection”, Proceedings of the 16th IEEE Photovoltaic Specialists Conference, 1982, pp 1092-1101
Combined Displays and Photovoltaic Elements
A number of patents describe methods of combining photovoltaic (PV) elements and displays in the same area.
The simplest method is to manufacture PV and display separately and place the PV behind the display. This is a workable method when the display is driven by a passive matrix, so that the display electronics do not obscure the PV. U.S. Pat. Nos. 6,518,944 (Doane; Kent Displays), 5,523,776 (Hougham; IBM), 7,206,044 (Li; Motorola) and patent application US20050117096 (Voloschenko; Motorola) describe this arrangement. This method has two disadvantages: since separate substrates are used for the display and the PV, the weight and thickness of the combined device are large; and when an active matrix is used in the display, the electronics of the active matrix absorb some light before it reaches the PV, reducing the PV efficiency.
An alternative arrangement is to place the PV and the display electronics in the same layer. This has the advantage that it may save manufacturing steps and so reduce cost. For example, each pixel may have silicon layers that in one part of the pixel act as a TFT and in another part of the pixel act as a photovoltaic element. U.S. Pat. Nos. 6,323,923 (Hoshino; Seiko-Epson) and 6,452,088 (Schmidt; Airify) describe this arrangement. Again, the efficiency of the PV is reduced in this arrangement because it occupies only part of the available area. The energy of light striking parts of the display area not covered by PV is lost.
Similarly, U.S. Patent Application No. 20070102035 (Yang) describes an arrangement where part of a transmissive display is obscured by a black mask with an integrated PV function. Again, the disadvantage is that because the PV occupies only a small part of the area, it can gather only a small part of the light energy falling on the device.
Two patent applications (US20090103165, US20090103161: Kothari; Qualcomm) describe devices where PV elements are combined with a MEMS display. In both cases, electronics on the top substrate drive a MEMS device, and PV elements are placed either outside the substrates or on the lower substrate. In the first case, the PV device partly obscures the display, and in the second case the display obscures the PV.
There are also patents where a PV element is combined with an organic LED display. An example is U.S. Patent Application No. 20090219273 (Nathan; Ignis Inc.). This patent also mentions a liquid crystal display as a possibility, but does not give details.